Introduction. In recent years, determination of soluble transferrin receptor levels has been emerging as a test that can reliably indicate iron deficiency in various states, and that is non-invasive and easy to use. The aim of this study was: to determine reference values of soluble transferrin receptor concentrations in serums in our population, to examine the reliability of this method in the diagnosis of anemia due to iron deficiency and associated iron deficiency in anemia accompanying malignant hemopathies, and to identify possible limitations of the test in certain conditions. Material and Methods. The prospective research included 86 patients with anemia: 46 patients with iron deficiency anemia, and 40 patients with malignant hemopathies. The control group consisted of 40 healthy persons aging over 18. Results. Ferritin values were reduced in 76.1% of patients, while higher levels of soluble transferrin receptors appeared in 100% of patients with iron deficiency anemia. In patients with reduced serum ferritin levels, the soluble transferrin receptor/log ferritin index was statistically significantly higher than in patients in whom ferritin concentration was in the normal range (p <0.001). ROC analysis of patients with iron deficiency anemia showed that the soluble transferrin receptor/log ferritin index (AUC 0.977) and levels of soluble transferrin receptors (AUC 0.931) occupied the largest area under the curve. The best diagnostic parameter for detecting iron deficiency in patients with malignant hemopathies by ROC analysis is the soluble transferrin receptor/log ferritin index (AUC 0.770). Conclusion. Soluble transferrin receptors are useful in the diagnosis of iron deficiency anemia, especially when ferritin values are not reduced. The calculation of soluble transferrin receptor/log ferritin index is even more reliable. In patients with malignant hemopathies, the associated iron deficiency could be best indicated by soluble transferrin receptor/log ferritin index.